California Structural Steel Contractor (C-51 License) Practice Exam

What is the ratio of oxygen to acetylene for complete combustion in oxyacetylene welding?

1:2 2:1 3:2 1:1

Correct Answer:

2:1

Explanation:

The key idea is how much oxygen is needed to burn acetylene completely. For complete combustion of acetylene (C2H2) with oxygen, the balanced reaction is: 2 C2H2 + 5 O2 -> 4 CO2 + 2 H2O This shows that 5 moles of O2 are required for 2 moles of C2H2, which is a ratio of 5:2 or 2.5:1 (oxygen to acetylene). In other words, the exact stoichiometric ratio is 2.5 parts oxygen per 1 part acetylene. Among the given options, 2:1 is the closest practical approximation used in oxyacetylene welding. It provides enough oxygen to achieve complete combustion in most welding contexts without leaving unburned fuel, whereas ratios lower than that would lead to incomplete combustion with soot or CO, and higher ratios would create a more oxidizing flame. So the best choice is the 2:1 ratio.

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